A laser beam source for generating and/or emitting coherent electromagnetic radiation generally includes a lasing medium constructed from a homogeneous body of solid state lasing material such as yttrium aluminum garnet (YAG) doped with neodymium (Ng:YAG) or neodymium doped glass. Known lasing mediums have many configurations such as a cylindrical configuration utilized in rod lasers. With rod lasers, however, during operation, thermal gradients are formed within the medium. The thermal gradients distort coherent light wavefronts transmitted therethrough and this distortion adversely affects the operation efficiency and quality of the beam.
Improvements have been made in medium configuration to minimize wavefront distortion. One example of an improved lasing medium configuration is disclosed in commonly assigned U.S. Pat. No. 4,740,983, entitled "Laser Apparatus for Minimizing Wavefront Distortion" to Azad, the disclosure of which is incorporated in its entirety herein by reference. The lasing medium of the laser device disclosed in the Azad patent generally has end surfaces, a hexagonal cross section and an interior surface with a circular cross section coaxial with a longitudinal axis of the medium. The interior surface of the Azad device defines an interior space for receiving a radiation pumping source, and the external side surfaces of the lasing medium are surrounded with a circumferential reflector. A prism is disposed on a side surface substantially adjacent each end surface of the medium for the introduction/extraction of coherent radiation. When operating the Azad device as an amplifier, the pumping source emits radiation which excites atoms of the lasing medium. Fluid coolant is flowed over the internal and side surfaces of the medium and a coherent light beam is introduced through one prism. The beam passes through the medium following a helical course and the beam totally internally reflects off the side surfaces of the medium. An amplified beam exits the medium through the other prism. Until now, utilizing prisms for the introduction/extraction of the beam was the preferred construction.
Introduction/extraction of the beam through the prisms, however, causes thermal gradients to be formed within the lasing medium during operation. The thermal gradients are caused by non-symmetric flashlamp pumping and liquid/gas cooling geometries due to the presence of the prisms. Specifically, the pumping lamp emits electromagnetic radiation which impinges uniformly over the entire area of the interior surface of the lasing medium. The electromagnetic radiation from the pumping source causes excitation of atoms within the lasing medium, and this excitation will be uniform throughout the medium except for at the location of the prisms. The non-uniform excitation at the prisms causes distortion of the beam. The prisms also cause non-uniform cooling geometries because the prisms alter the otherwise uniform geometry of the medium. Although the distortion of the beam wavefront due to the prisms has been described with reference to the medium operating as an amplifier, distortion of the beam wavefront also occurs when the medium operates in other modes, such as an oscillator.